1. FIELD OF THE INVENTION
The present invention relates to a sheet feeding apparatus for feeding a sheet such as writing or manuscript paper for use in a printer, copying machine, facsimile machine, or printing machine.
2. DESCRIPTION OF THE RELATED ART
FIGS. 25-27 show a conventional sheet feeding apparatus, wherein FIG. 25 is a side view of a thermal printer, and a sheet in a thermal printer is shown in FIGS. 26 and 27 as a perspective and a side view, respectively. With reference to these figures, a sheet 30 is fed one by one from a sheet feeding mechanism 15. A leading end portion of the sheet 30 is inserted into a clamper 10, and a clamper closing mechanism (not shown) then closes the clamper 10 so as to hold the sheet 30.
A driving roller 1 (sheet-feeding driving roller) feeds a sheet and acts as a platen roller. A bridge 10a is provided between a pair of timing belts 3, 3 in such a manner that it is parallel to the sheet-feeding driving roller 1. The clamper 10 described above is fixed to the bridge 10a. A pair of first pulleys 2 are freely rotatable about the axis of the sheet-feeding driving roller 1. A pair of second pulleys 4, 4 are driven by a second motor 12 via a torque limiter 13.
With this arrangement, the timing belts 3, 3 are driven by the second pulleys 4, 4 to travel along a circulating path. Following this movement of the timing belts 3, the clamper 10 moves in the direction shown by arrow B (see FIG. 26). The running speed V2 of the clamper 10 is determined by the number of revolutions per minute N2 of the second pulleys 4, 4 which is in turn determined by the constant number of revolutions per minute M of the second motor 12 as long as no shipping occurs at the torque limiter 13.
With the arrangement described above, the clamper 10 moves passing by the first pulleys 2, 2, second pulleys 4, 4 and third pulleys 5, 5, thus returning to its starting position. During this circulating movement, the sheet 30 held by the clamper 10 is pressed against a thermal head 9 by means of the sheet-feeding driving roller 1. As a result of this, the color of an inking sheet 6 carrying color inking materials is transferred to the sheet 30.
In the case of color copying, the process is performed as follows. Assuming that the copying is done for Y (yellow) first, then M (magenta), C (cyan), and finally BK (black), first of all the leading end of the inking sheet 6 of Y is positioned, and the leading end of the sheet 30 is also positioned with the aid of a sensor PH1 which detects the leading end of the sheet. Then, the thermal head being pressed against the sheet-feeding driving roller 1, the inking sheet 6 and the sheet-feeding driving roller 1 as well as the clamper 10 are driven to move. During this process, a thermal head driver (not shown) heats the thermal head 9 according to the printing data so as to perform printing.
When printing is completed for one color, the thermal head 9 is separated from the sheet-feeding driving roller 1. Then, positioning of the leading end of the inking sheet 6 is performed for M (magenta) and printing is carried out in the same way as in the case of Y (yellow). In this printing process, the sheet 30 is circulated again passing by each of the pulley 2, 2, 4, 4, 5, and 5.
The same procedure is repeated to print the colors C (cyan) and BK (black).
During printing process for each color, the thermal head 9 is pressed against the sheet-feeding driving roller 1 via the sheet 30. Therefore, the sheet 30 is carried according to the rotation of the sheet-feeding driving roller 1 which is driven by a driving motor 11. In other words, the sheet 30 is carried at a constant speed V1 which is determined by the rotational speed of the sheet-feeding driving roller 1. As a result, the clamper 10 holding the sheet 30 also runs at the same speed V1.
While this clamper running speed V1 represents the running speed of the sheet 30 and the clamper 10 during the printing process, the previously described clamper running speed V2 represents the speed when no printing process is performed. The clamper running speed V2 is set to a value faster than the sheet running speed V1. The difference in speed between V1 and V2 is absorbed by slipping of the torque limiter 13. This slipping occurs such that a predetermined magnitude of torque determined by the torque limiter 13 is applied to the clamper 10 via the second pulleys 4, 4, and the timing belts 3, 3. This means that during the printing process, the clamper 10 pulls the sheet 30 with a tension of a predetermined value.
In such a sheet feeding apparatus for a thermal printer described above, the sensor PH1 disposed in a sheet feeding path detects the sheet feeding condition. When the sensor PM1 detects that the sheet 30 has arrived at the starting position of printing, the thermal heads 9 starts printing. However, even if the starting position is given accurately, is difficult to perform accurate printing at desired positions along the whole length of a sheet.
This problem occurs because of the fluctuation of the tension applied to the sheet 30 which results from the fluctuation of torque of the torque limiter 13, tension of the inking sheet 6, and the coefficient of friction of the sheet-feeding driving roller 1. That is to say, when the sheet 30 is bitten by or fed between the sheet-feeding driving roller 1 and the thermal head 9 at their contacting position, and the sheet 30 is carried according to the rotation of the sheet-feeding driving roller 1, there may occur slight slippage between the sheet 30 and the sheet-feeding driving roller 1 due to a difference in the sheet tension between the areas before and after the sheet-feeding driving roller 1, and due to the fact that the amount of the slippage changes depending on a change in the tension applied to the sheet 30.
In the case of high density printing for the whole sheet, the temperature of the sheet-feeding driving roller rises, which results in an increase in the diameter of the sheet-feeding driving roller 1, which further results in an increase in the sheet feeding length.
In particular, in the case of color printing, if the above phenomena introduce variations of printing positions between each color to be composited together, degradation of printing quality occurs due to the registration error between different colors.
In a region near the starting position, it is possible to achieve small registration errors between each of colors less than a maximum tolerance, because the positioning of the starting point of each color can be done accurately enough as described earlier.
However, the error in the amount of feeding of the sheet is accumulated and the error can become large in the area near the trailing end thereof, thus noticeably large registration errors between colors may appear. In particular, in the case of a large-sized sheet such as a standard A3 size or larger, it is difficult to achieve small registration errors between colors less than a maximum tolerance along the whole sheet.
The problem described above occurs because the positioning of the sheet 30 is carried out only at the leading end thereof when feeding operation starts and because the position of the sheet 30 during the feeding process cannot be detected. If the position of the sheet can be detected during the feeding process and if a deviation of the sheet position from the reference position can be determined, then it becomes possible to correct the printing position or the amount of feeding. Therefore, the most important issue is to detect the position of the sheet during the feeding process.
In general, rubber is used as a material forming the surface of the sheet-feeding driving roller 1. Aging of the rubber results in another problem that the printing length changes due to variations in the diameter of the sheet-feeding driving roller 1.